In the theme of html, you want to chagne font or icon. Look at this site, Font Awesome library to see the available icons.

The Font Awesome icons allow you to adjust their size by adding fa-2x, fa-3x and so forth as a class to the icon to adjust their size to two times or three times original size

let’s see an simple example with

attach icon 

<i class="fa fa-paperclip"></i> normal size (1x)
<i class="fa fa-paperclip fa-lg"></i> fa-lg
<i class="fa fa-paperclip fa-2x"></i> fa-2x
<i class="fa fa-paperclip fa-3x"></i> fa-3x
<i class="fa fa-paperclip fa-4x"></i> fa-4x
<i class="fa fa-paperclip fa-5x"></i> fa-5x

normal size (1x) fa-lg fa-2x fa-3x fa-4x fa-5x

alert

Note :

<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note: </b> Add your note here </div>

Tip :

<div class="alert alert-success" role="alert"><i class="fa fa-check-square-o"></i> <b>Tip: </b> Add your tip here </div>

important :

<div class="alert alert-warning" role="alert"><i class="fa fa-warning"></i> <b>Important: </b> Add your important here </div>

warning :

<div class="alert alert-danger" role="alert"><i class="fa fa-exclamation-circle"></i> <b>Warning: </b> Add your warning here </div>

Reference